Oil-soluble zinc dithiophosphates are used as compounding agents or additives in lubricating oils to inhibit engine wear, to inhibit oxidation of the lubrication oil, and to inhibit engine part corrosion. It is highly desirable for such oil-soluble zinc dialkyldithiophosphates to provide the aforesaid properties to the lubricating oil without affecting the appearance characteristics of the formulated lubricant composition even though the performance characteristics of the lubricant composition may not be affected noticeably.
As is well known, oil-soluble zinc dialkyldithiophosphates are conventionally prepared by reacting monohydric alcohols or phenols with phosphorus pentasulfide, usually in a mole ratio of 4:1, alcohol to phosphorus pentasulfide, in the presence of a diluent oil at a temperature within the range of from about 70.degree. C. to about 135.degree. C. to form partial esters of dithiophosphoric acid, a green odoriferous liquid. The so-obtained partial esters are then neutralized with zinc oxide at a temperature within the range of from about 70.degree. C. to about 135.degree. C. to form a zinc dialkyldithiophosphate, typically a light yellow liquid.
It has been found that, upon storage, the zinc dialkyldithiophosphate prepared as described often will acquire a dark objectionable color, often of a brownish hue. The dark color is objectionable as it affects the marketing of the zinc dialkyldithiophosphates. The dark product has been considered as inferior to product of a light color.
It has long been known that the metal salts of partial esters of dithiophosphoric acids can be decolorized by incorporation of a small amount of a trialkylolamine whose alkylol groups contain 1 to 4 carbon atoms each. U.S. Pat. No. 2,983,742 teaches that excellent results have been obtained with triethanolamine but other amines of the class disclosed can be used. Amounts can be used of from 0.25 to 1.0 percent by weight. U.S. Pat. No. 3,361,668 teaches and claims a lubricating composition containing light-colored metal phosphorodithioates wherein the said phosphorodithioate is prepared by reacting phosphorus pentasulfide with a mixture of from about 95% to 99.95% by weight of a monohydroxy alcohol or phenol having from 1 to 30 carbon atoms and from about 0.05% to 5.0% by weight of an alkyl amine, cycloalkylamine or heterocyclic amine having up to about 20 carbon atoms. In an alternative procedure, the phosphorodithioic acid diester is first prepared and a small amount of a suitable amine is then added.
In the prior art, it has been postulated that the cause of the dark color in metal salts of dithiophosphate esters is the presence of traces of heavy metals, principally iron, in the phosphorus pentasulfides. U.S. Pat. No. 2,983,742, Boba, et al, considers that addition of a trialkylolamine forms complexes with traces of the heavy metal compounds in the dithiophosphate esster salt that are more stable than the uncomplexed metal compounds. However, Boba, et al, U.S. Pat. No. 2,983,742 indicate this postulate is somewhat negatived by the fact that other agents capable of forming complexes with heavy metal compounds do not produce equivalent results. Wiese, U.S. Pat. No. 3,361,668, teaches that commercially prepared phosphorodithioic acid diesters are sufficiently strong acids to be corrosive to metals and the use of special corrosion-resistant equipment is generally required in their manufacture. Wiese U.S. Pat. No. 3,361,668 teaches that the corrosiveness of the phosphorodithioic acid esters upon mild steel is shown to be reduced in the presence of an amine-reacted product of a phosphorodithioic acid ester. Wiese U.S. Pat. No. 3,361,668 accordingly teaches that the amine-reacted product of a phosphorodithioic acid is less reactive to iron compounds and retains its light color as prepared.
It has now been found unexpectedly that acid salts comprising zinc dialkyldithiophosphates substantially retain their original light color despite exposure to elevated temperatures for prolonged periods of time upon the addition of color stabilizers in an amount not greater than 1.0 (wt)% of the zinc dialkyldithiophosphate metal salt in concentrations of from 0.0001 to 0.5 mole of stabilizer per mole of zinc dialkyldithiophosphate metal salt. A preferred range is 0.002 to 0.05 mole of stabilizer per mole of zinc dialkyldithiophosphate. The color stabilizers are added after the dialkyldithiophosphoric acid diester is neutralized with zinc oxide to prepare the zinc dialkyldithiophosphate. Some of the color stabilizers are preferably added to the dialkyl dithiophosphoric acid diester after it is neutralized with zinc oxide. The color stabilizers comprise hydrogen sulfide, olefins, mercaptans, epoxides, phosphites, and phosphones.
Although the mechanism has not been definitely established by which the color stabilizers cause the zinc dialkyldithiophosphate to retain a light color, it is believed that the addition of the aforementioned color stabilizers decreases the rate of oxidation of the zinc dialkyldithiophosphate under conditions of heat and/or prolonged storage; nor has the darkening of the zinc dialkyldithiophosphate been definitely established as having been caused by the presence of oxygen and consequent oxidation of the dithiophosphate ester. It is also considered that traces of other compounds may be present which react with zinc dialkyldithiophosphate to cause a darkening of the ester.